1. Field of the Invention
This invention relates generally to determining analyte concentrations in material samples.
2. Description of the Related Art
Millions of diabetics draw samples of bodily fluid such as blood on a daily basis to monitor the level of glucose in their bloodstream. This practice is called self monitoring, and is commonly performed using one of a number of reagent based glucose monitors. These monitors measure glucose concentration by observing some aspect of a chemical reaction between a reagent and the glucose in the fluid sample. The reagent is a chemical compound that is known to react with glucose in a predictable manner, enabling the monitor to determine the concentration of glucose in the sample. For example, the monitor can be configured to measure a voltage or a current generated by the reaction between the glucose and the reagent. A small test strip is often used to hold the reagent and to host the reaction between the glucose and the reagent. Reagent based monitors and test strips suffer from a variety of problems and also have limited performance.
Problems and costs relating to reagent arise during manufacture, shipment, storage and use of the reagent containing test strips. Costly and demanding quality control strategies must be incorporated into the test strip manufacturing processes to assure that the strips ultimately function properly. For example, a manufacturing lot specific calibration code must be determined through blood or equivalent testing before the strips can be released for consumer sale. The diabetics using the reagent based monitors must often enter this calibration code into the monitor to ensure that the monitor accurately reads the concentration of glucose in a sample placed on the strip. Naturally, this requirement leads to errors in reading and entering the calibration code, which can cause the monitor to make dangerously inaccurate readings of glucose concentration.
Reagent based monitor test strips also require special packaging during shipment and storage to prevent hydration of the reagent. Premature hydration affects the manner in which the reagent reacts with glucose and can cause erroneous readings. Once the test strips have been shipped, they must be stored by the vendor and user within a controlled storage temperature range. Unfortunately, the multitude of users are often unable to follow these protocols. When test strips and their reagents are not properly handled and stored, erroneous monitor readings can occur. Even when all necessary process, packaging and storage controls are followed, the reagents on the strips still degrade with time, and thus the strips have a limited shelf life. All these factors have led consumers to view reagent based monitors and test strips as expensive and troublesome. Indeed, reagent based test strips would be even more expensive if they were designed to be made simpler and completely fail safe.
The performance of reagent based glucose monitors is limited in a number of respects related to reagents. As discussed above, the accuracy of such monitors is limited by the sensitive nature of the reagent, and thus any breakdown in the strict protocols relating to manufacture, packaging, storage and use reduces the accuracy of the monitor. The time during which the reaction occurs between the glucose and the reagent is limited by the amount of reagent on the strip. Accordingly, the time for measuring the glucose concentration in the sample is limited as well. Confidence in the reagent based blood glucose monitor output can be increased only by taking more fluid samples and making additional measurements. This is undesirable because it doubles or triples the number of painful fluid removals. At the same time, reagent based monitor performance is limited in that the reaction rate limits the speed with which an individual measurement can be obtained. The reaction time is regarded as too long by most users.
Generally, reagent based monitors are too complex for must users, and have limited performance. Additionally, such monitors require users to draw fluid multiple times per day using sharp lances which must be disposed of carefully.